1. Field
The present invention relates generally to a flexible optical circuit for use in a fiber amplifier and, in particular, to a flexible circuit that is temperature-controlled.
2. Description of Related Art
An erbium-doped fiber amplifier (EDFA) is basically an optical repeater device that functions to boost the amplitude of optical signals traveling through a fiber optic communications system. In particular, the EDFA incorporates a variety of components including a laser diode, a multiplexer and an optical fiber which is doped with the rare earth element erbium. The laser diode emits light having an infrared wavelength of 980 nm or 1480 nm that is passed through the multiplexer into the erbium-doped optical fiber. The emitted light excites the erbium atoms in the optical fiber. Then when an input optical signal having a wavelength of between 1530 nm and 1620 nm passes through the multiplexer and enters the optical fiber it stimulates the excited erbium atoms to emit photons at the same wavelength as the input optical signal. This action amplifies the input optical signal to a higher power by effectively boosting the amplitude of the input optical signal. Examples of two traditional EDFAs 100 and 200 are briefly discussed below with respect to FIGS. 1 and 2.
Referring to FIG. 1 (PRIOR ART), there is a block diagram illustrating the basic components of a traditional EDFA 100. The EDFA 100 includes a variety of components including a laser diode 102, a multiplexer 104 and a custom-designed bobbin 106 that holds a predetermined length of erbium-doped optical fiber 108. The optical fiber 108 which can be relatively long (e.g., ˜50 m) is wrapped around the bobbin 106 before being placed in a package 110. The package 110 contains the various components that make-up the EDFA 100 including the laser diode 102, the multiplexer 104 and the bobbin 106. In operation, the EDFA 100 receives an input optical signal that is coupled by the multiplexer 104 along with the light from the laser diode 102 into the erbium-doped optical fiber 108 which becomes excited by the light from the laser diode 102 and outputs an amplified optical signal.
Unfortunately, there are a number of disadvantages associated with using the bobbin 106 to hold the optical fiber 108. First, the bobbin 106 needs to be custom designed so it can fit within the package 110. Secondly, the bobbin 106 itself is bulky and restricts the overall outline of the package 110. Thirdly, the optical fiber 108 may be stressed if the optical fiber 108 is wrapped too tightly around the bobbin 106. As such, the custom-designed bobbin 106 delays and adds complexity to the design of the EDFA 100 and can also adversely affect the operability of the EDFA 100.
Referring to FIG. 2 (PRIOR ART), there is a block diagram illustrating the basic components of another traditional EDFA 200. The EDFA 200 includes a variety of components including a laser diode 202, a multiplexer 204 and a predetermined length of erbium-doped optical fiber 206 that is held together by a fastener 208 including, for example, wire, string, tape, or glue (shown and described below as three wires/strings 208). Prior to being inserted into the EDFA 200, the optical fiber 206 which can be relatively long (e.g., ˜50 m) is wrapped around a custom-designed fixture 210 (e.g., bobbin)(see exploded view). Once the desired length of optical fiber 206 is wrapped around the fixture 210, then the optical fiber 206 is removed from the fixture 210 and the loose coil of optical fiber 206 is contained by the wire/string 208 (see exploded view). The optical fiber 206 that is held together by the wire/string 208 is then placed in a package 212. The package 212 contains the various components that make-up the EDFA 200 including the laser diode 202, the multiplexer 204 and the optical fiber 206. In operation, the EDFA 200 receives an input optical signal that is coupled by the multiplexer 204 along with the light from the laser diode 202 into the erbium-doped optical fiber 206 which becomes excited by the light from the laser diode 202 and outputs an amplified optical signal.
Unfortunately, there are a number of disadvantages associated with using the fixture 210 to wrap the optical fiber 206 and for using the wire/string 208 to contain the optical fiber 206. First, the fixture 210 needs to be custom designed such that the coil of optical fiber 208 has the desired diameter so it can fit within the package 212. Secondly, the optical fiber 206 may be stressed if the optical fiber 206 is wrapped too tightly around the fixture 210 and also difficult to remove. Thirdly, the optical fiber 206 may be stressed if the wire/string 208 is wrapped too tightly around the optical fiber 206. As such, the custom-designed fixture 210 delays and adds complexity to the design of the EDFA 200 and the use of wire/string 208 to hold the loose coil of optical fiber 206 can adversely affect the operability of the EDFA 200. A new flexible optical circuit for use in EDFAs was first described in co-owned and co-pending U.S. patent application Ser. No. 10/127,089, filed Feb. 26, 2001, and entitled Flexible Optical Circuit For Use In An Erbium-Doped Fiber Amplifier And Method For Fabricating The Flexible Optical Circuit, which is incorporated by reference herein.
It is known in the art that the gain of an optical amplifier is dependent upon the temperature of the optical fiber. As the temperature of the fiber fluctuates, so does the gain and noise figure. However, it is desired for amplifier gain and noise figure to be consistent and therefore predictable during operation. Thus, maintaining the temperature of the fiber at a constant level irrespective of ambient temperature is desirable.
Techniques for compensating for temperature variations in EDFAs have been used with varied results. One could alter the power of the “pump” laser source in response to temperature to modify the population inversion. This provides a flat gain curve over temperature and has the advantage of requiring no external components. However, this results in a change in total output power which is unacceptable in most designs. The input signal power could also be altered with respect to temperature to modify the population inversion by coupling to the EDFA a variable optical attenuator. The disadvantage of this technique is that total gain of the EDFA would have to be increased in order to compensate for loss resulting from the attenuator.
A variable gain equalizer, either temperature-dependent or manually controlled, may be used to vary gain as necessary. These devices are expensive, increasing the cost of the system, and their use also results in signal loss. Also, gain-flattening filters (“GFF”s) have been used to attempt to compensate for temperature-induced variation in gain, however, such filters are themselves temperature dependent making them inappropriate for temperature compensation of the overall gain.
A final technique of overcoming variation in gain due to temperature is heating the optical fiber inside the EDFA. However, current methods of maintaining constant temperature of an optical fiber used in erbium-doped fiber amplifiers are bulky, and increase thermal resistance and assembly time. For example, FIG. 3A (PRIOR ART) shows an optical circuit 300 employing a specially designed bobbin 301 to hold the optical fiber 304 in place similar to that used in the EDFA shown in FIG. 1. To manage temperature of the optical fiber 304, a heating element 307 is mechanically attached to the bobbin 301. FIG. 3B (PRIOR ART) shows the optical circuit 300 with specially designed bobbin 301. To manage temperature of the optical fiber 304, a heating coil 308 is wrapped around bobbin 301. In addition, to the lengthy time to produce such a temperature-controlled optical fiber for use in an EDFA, each time a new amplifier is designed, along with a new bobbin or spool, a new heating element or coil must also be developed increasing design time. Moreover, the heating element would increase system power consumption.
Thus, a new flexible optical circuit that allows for temperature control of the erbium-doped optical fiber is needed that is less bulky and reduces assembly time, making production more efficient.